To follow up on this - managed to get the motion snapshot functionality working by using additionally the Hikvision app ([App] Hikvision).
In addition to ONVIF (per above post) to enable “Notify Surveillance Center” option for the motion events in DVR settings. For my DVR the option is available under Configuration->Event->Basic Event->Linkage Method. Once enabled, the Hikvision app will capture these events from DVR, and then they may be used as triggers for motion snapshot updates.
As my cameras are analog (TVI) ones I cannot add them to Homey directly and need to use both Hikvision and ONVIF Camera apps to capture events and snapshot images from DVR. This does not work 100% perfect as you need to play with timings to get a meaningful picture after motion event occurs, but on the upside you may use other event alerts (line crossing, intrusion etc.) available from DVR to trigger snapshots.
